High-speed low-jitter communication system

ABSTRACT

Communication apparatus and techniques, such as for optical communication, can include providing a reference frequency derived from an atomic energy level transition or a molecular energy level transition, generating at least two specified optical carrier signals at least in part using the reference frequency, coherently modulating the specified optical carrier signals using respective baseband information signals to provide respective coherently-modulated optical subcarriers. A combined optical information signal comprising the optical subcarriers can be transmitted to a receiver, such as via a fiber optic cable. In an example, a received optical information signal can be optically Fourier transformed to provide respective coherent outputs, which can be coherently downconverted.

CLAIM OF PRIORITY

This patent application claims the benefit of priority, under 35 U.S.C.Section 119(e), to Wilkinson et al., U.S. Provisional Patent ApplicationSer. No. 61/594,655, titled “High-Speed Low-Jitter CommunicationSystem,” filed on Feb. 3, 2012, which is hereby incorporated byreference herein in its entirety.

BACKGROUND

Orthogonal frequency division multiplexing (OFDM) is an example of abroader class of multi-carrier data transmission techniques. OFDM can beused as a physical-layer interface in wireless communications, such asto combat multipath fading or frequency-selective fading. For example,OFDM has been incorporated into numerous wireless communicationstandards such as including IEEE 802.11a/g (“WiFi”), HiperLAN2, and IEEE806.16 (“WiMAX”), as well as in the digital audio and video broadcastingstandards, DAB and DAB-T, used in Asia, Australia, and Europe.

Overview

A high-speed data stream can be transmitted, such as optically, usingtwo or more subcarrier channels, such as using an OFDM technique. Thesubcarrier channels can operate at a lower transmission rate and incombination can provide the high-speed data stream. Each subcarrier canbe at a different frequency and together the subcarriers can form aregularly spaced frequency comb, such as similarly achieved using densewavelength division multiplexing (DWDM). However, in OFDM there is amathematical orthogonality between subcarriers, unlike DWDM. Such anorthogonality property allows spectral overlap between subcarriers suchas without causing inter-channel interference (ICI), as shown in theexample of FIG. 1, such as resulting in improved spectral efficiency,and hence supporting an increased data rate as compared to DWDM.

Generally, generating an OFDM information signal includes modulatingboth amplitude and phase of the subcarrier signal (e.g., complexmodulation), and receiving an OFDM information signal includescoherently demodulating the subcarrier signal. Thus, varioussynchronization techniques can be used to synchronize an OFDM receiverwith an OFDM transmitter to maintain such coherency. In an example, oneor more portions of an OFDM transmitter can be locked to a referencefrequency provided by a first local optical oscillator included as aportion of the OFDM transmitter apparatus. Similarly, one or moreportions of an OFDM receiver can be locked to a reference frequencyprovided by second local optical oscillator included as a portion of theOFDM receiver apparatus.

In an example, a local optical oscillator can be configured to providethe reference frequency at least in part using one or more of atomicenergy level transition or a molecular energy level transition. Use ofsuch a local optical oscillator can decrease an amount of communicationoverhead consumed in periodically training or synchronizing an OFDMtransmitter with an OFDM receiver. For example, a duration over whichthe transmitter and receiver can remain synchronized can be extended toseveral seconds between training intervals.

In an example, such as for optical communication, apparatus andtechniques can include providing the reference frequency derived from anatomic energy level transition or a molecular energy level transition,generating at least two specified optical carrier signals at least inpart using the reference frequency, and coherently modulating thespecified optical carrier signals using respective baseband informationsignals to provide respective coherently-modulated optical subcarriers.A combined optical information signal comprising the optical subcarrierscan be transmitted to a receiver, such as via a fiber optic cable. In anexample, a received optical information signal can be optically Fouriertransformed to provide respective coherent outputs, which can becoherently downconverted.

In an example, such communication as discussed above (e.g.,multi-carrier OFDM communication) need not be optical. A communicationchannel can include a microwave link, a radio-frequency (RF) link, aconductive communication link such as using one or more of a coaxialconductor arrangement, a twisted pair arrangement, a conductivewaveguide configuration, or can include communication using one or moreother media or techniques. For example, an information signal (e.g., anOFDM information signal), such as including one or morecomplex-modulated subcarriers, can be provided such as using anelectronic arbitrary waveform generator or other electronic source usinga reference frequency provided by a first local optical oscillator. Uponreceipt, such an information signal can then be coherently demodulated,such as using one or more of a mixer or an analog-to-digital converter,such as referenced to a second local optical oscillator. As in otherexamples above, the first and second local optical oscillators canprovide respective reference frequencies derived from an atomic energylevel transition or a molecular energy level transition.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which are not necessarily drawn to scale, like numeralsmay describe similar components in different views. Like numerals havingdifferent letter suffixes may represent different instances of similarcomponents. The drawings illustrate generally, by way of example, butnot by way of limitation, various embodiments discussed in the presentdocument.

FIG. 1 illustrates generally a comparison between wavelength divisionmultiplexing (WDM) and orthogonal frequency division multiplexing(OFDM).

FIG. 2 illustrates generally an example of a communication system thatcan include first and second local optical oscillators, a complexmodulator, and a coherent demodulator.

FIG. 3 illustrates generally an architectural view of an illustrativeexample of communication system.

FIGS. 4A and 4B illustrate generally examples of coherent demodulatorsthat can include a heterodyne configuration in FIG. 4A and a homodyneconfiguration in FIG. 4B.

FIG. 5 illustrates generally an example of at least a portion of anoptical oscillator that can be referenced to an atomic energy leveltransition or a molecular energy level transition and FIG. 6 illustratesgenerally an example of a frequency comb generator, such as referencedto a signal provided by the apparatus of the example of FIG. 5.

DETAILED DESCRIPTION

FIG. 1 illustrates generally a comparison between wavelength divisionmultiplexing (WDM) and orthogonal frequency division multiplexing(OFDM). In DWDM a frequency guard-band is inserted between eachsubcarrier to prevent inter-channel interference (ICI). In contrast,OFDM subcarriers are orthogonal and can overlap significantly withoutcausing ICI. In OFDM data transmission is achieved by parallelmodulation two or more subcarriers, and can include a large number ofsubcarriers. Prior to transmission, the subcarriers are multiplexed, aprocess mathematically equivalent to performing the inverse discreetFourier transform (IDFT) of the subcarrier group. The resulting timedomain baseband signal can be represented by:

$\begin{matrix}{{s(t)} = {\sum\limits_{n = {- \infty}}^{\infty}{\sum\limits_{k = 1}^{N_{sc}}{{\Pi( {t - {nT}_{s}} )}c_{nk}{\mathbb{e}}^{{{j2\pi}{({t - {nT}_{s}})}}f_{k}}}}}} & {{EQN}.\mspace{14mu} 1} \\{{\Pi(t)} = \{ {{\begin{matrix}{1,} & ( {0 < t \leq T_{s}} ) \\{0,} & ( {t > T_{s} \parallel t \leq 0} )\end{matrix}f_{k}} = \frac{k - 1}{T_{s}}} } & {{EQN}.\mspace{14mu} 2}\end{matrix}$where c_(nk) and f_(k) can represent the n^(th) information symbol andfrequency of subcarrier k, respectively, T_(s) can represent the symbolperiod, and N_(sc) can represent the total number of subcarriers.Calculation of the inner-product between two arbitrary subcarriers, canbe represented by,

$\begin{matrix}{{{\frac{c_{nk}c_{lk}^{*}}{T_{s}}{\int_{0}^{T_{s}}{{\mathbb{e}}^{{{j2\pi}{({f_{k} - f_{l}})}}t}{\mathbb{d}t}}}} = {\frac{c_{nk}c_{lk}^{*}}{T_{s}}\frac{\sin( {{\pi( {f_{k} - f_{l}} )}T_{s}} )}{\pi( {f_{k} - f_{l}} )}}},} & {{EQN}.\mspace{14mu} 3}\end{matrix}$and can demonstrate that if the condition,

$\begin{matrix}{{f_{k} - f_{l}} = {m\frac{1}{T_{s}}}} & {{EQN}.\mspace{14mu} 4}\end{matrix}$is satisfied for any integer m, then the two subcarrier are orthogonalto each other. This means that if subcarriers are spaced by multiples ofthe inverse data modulation rate, significant spectral overlap ispossible without causing inter-carrier interference. Spectral efficiencycan be maximized when the modulation rate is equal to the spacingbetween subcarrier channels, (e.g., when m=1).

Subcarrier spectral overlap makes the separation of channels at thereceiver using conventional frequency filtering (e.g. narrowbandfiltering of each subcarrier) impractical. Rather, subcarriers can bedemultiplexed via a discrete Fourier transform (DFT) of the time domainsignal. This can be carried out electronically if the received signal isdetected at a sufficiently high bandwidth, or it can be carried outoptically prior to detection. Optical DFT techniques can remove theelectronic bottleneck of high speed detection and subsequent DFTcalculation.

In addition to its improved spectral efficiency over DWDM, two strengthsof OFDM over other transmission techniques, such as DWDM ortime-division multiplexing (TDM), include OFDM's inherent robustness tochromatic and polarization mode dispersion. For example, polarizationmode dispersion can be a particularly large impediment to wide-spreaddata transmission at rates of 40 gigabits per second (Gb/s) and higherusing in-ground fiber optics. Mitigation of these channel impairmentscan be accomplished via the insertion of a temporal guard interval ofduration that can be represented by Δ_(G), satisfying the relation,

$\begin{matrix}{{{\frac{c}{f^{2}}{D_{t}}N_{sc}\Delta\; f} + {DGD}_{\max}} \leq \Delta_{G}} & {{EQN}.\mspace{14mu} 5}\end{matrix}$where c can represent the speed of light, f can represent the centersubcarrier frequency, D_(t) can represent the total accumulatedchromatic dispersion in units of ps/pm, Δf can represent the frequencyspacing between subcarriers, and DGD_(max) can represent a budgeteddifferential group delay (e.g., a maximum), which, for example, can beabout 3.5 times larger than the mean polarization mode dispersion. In anexample, satisfaction of the condition of the example above can insurethat fiber dispersion does not lead to temporal overlap between symbols.

FIG. 2 illustrates generally an example of a communication system 100that can include first and second local optical oscillators 102A through102B, a complex modulator 106, and a coherent demodulator 110. In anexample, a baseband information signal can be provided to an input 104of a complex modulator 106. The complex modulator 106 can besynchronized or otherwise “locked” to a first reference frequencyprovided by a first local optical oscillator 102A. The complex modulator106 can be configured to modulate one or more of an amplitude or a phaseof one or more carrier signals, such as to provide an OFDM informationsignal coupled to a coherent demodulator 110 via a communication channel108.

The communication channel 108 can include one or more of an opticalcommunication link, a wireless electromagnetic communication link, or aconductive communication link. The coherent demodulator 110 can recoverthe baseband information signal (or a detected approximation of thebaseband information signal), such as provided by a baseband informationoutput 112. In order to demodulate the information signal provided bythe complex modulator 106, the coherent demodulator 110 can be coupledto a second local optical oscillator 102B, and the coherent demodulator110 can be synchronized or can otherwise use a second referencefrequency provided by the second local optical oscillator 102B. Forexample, the coherent demodulator can include one or more mixers, andthe one or more mixers can be coupled to respective local oscillatorsderived from the second reference frequency provided by the second localoptical oscillator 102B.

In an example, if both the first and second optical oscillators providerespective reference frequencies derived from an atomic energy leveltransition or a molecular energy level transition, a frequency offsetbetween the first reference frequency and the second reference frequencycan be specified to provide a symbol error rate of received informationbelow a specified threshold for a duration of at least one second ormore of information transfer without requiring a resynchronizationbetween the transmitter and the receiver. In this manner, as discussedbelow, a greater overall data transfer rate is possible because lesscommunication overhead is needed in sending synchronization informationvia the communication channel 108 rather than useful data.

FIG. 3 illustrates generally an architectural view of an illustrativeexample of communication system 300 (e.g., a transceiver). In FIG. 3,connections that can be electrical are shown as broken lines, andconnections that can be optical are shown with solid lines. Multiplesubcarriers can be generated by an optical carrier generator 314 (e.g.,optical frequency comb phase-locked to a precision frequency reference302, such as an RF source such as derived from a local opticaloscillator). The frequency spacing between subcarriers can establish anupper limit upon data transmission rate per subcarrier such as tomaintain channel orthogonality. Electronic detector speeds can establishan upper limit on channel spacing, and optical waveguide sizerestrictions and resolution requirements can establish a correspondinglower limit. In an example, the optical carrier generator 314 caninclude one or more of a mode-locked laser, a phase-modulated CW laser,a recirculating frequency shifter, or one or more other techniques orapparatus can be used, such as to provide an optical frequency combcomprising optical subcarriers. The subcarriers can be demultiplexedprior to data modulation, such as using an arrayed waveguide grating(AWG), or using one or more other techniques. For example, by selectingan AWG with a free spectral range equal to a multiple of the spacingbetween subcarriers, the number of independent data streams can bechosen.

Following subcarrier demultiplexing, independent data channels can becoherently modulated such as using a complex modulator 306. In anexample, a complex modulator driver can be coupled to a basebandinformation source 304 to provide baseband information signals such asfor driving the in-phase and quadrature inputs of a parallel array ofI/Q modulators. The array can include a respective complex modulator 306for each respective subcarrier. The OFDM time domain signal can beformed by combining the subcarriers into a single multiplexed opticalinformation signal using an optical combiner (e.g., an opticalmultiplexer), such as including a second arrayed waveguide grating, orother apparatus. This combining can be treated as equivalent to takingthe inverse discrete Fourier transform of the subcarrier group, such asto provide an optical information signal output 316A for transmissionthrough an optical fiber.

In an example, such as at a far end of such an optical fiber, orotherwise at a receiving end of the communication link at a receiverinput 316B, the subcarriers can be coherently detected, such as using anoptical mixer 310 (e.g., an optical hybrid) coupled to respectivebalanced detectors, and then digitally sampled.

A discrete Fourier transform (DFT), such as provided by an opticalFourier transform module 320, can be used to demultiplex and recover theindividual subcarriers from the optical information signal. The temporalwindow over which the receiver-end DFT is carried out is generallysynchronized to the symbol arrival rate and symbol duration. A frequencyoffset between the transmitter LO and the local oscillator used in thereceiver's optical hybrids can be measured and compensated, such as viadigital signal processing.

In an example, one or more optical carriers can be coupled from atransmitter block 330 of the system 300 to one or more portions of areceiving block 340 of the system 300, such as using an optical coupling318. In addition, or instead, in an example, one or more portions of thereceiving block 340 can be electrically coupled to the local opticaloscillator 302, such as to use one or more reference frequenciesprovided by the local optical oscillator 302 (e.g., the local opticaloscillator 302 can transfer the stability of its optical oscillator intothe RF frequency range for use by other portions of the system 300).

In one approach, initial synchronization can be performed using aSchmidl-Cox technique, such as using pilot-tones and test symbols, butresulting in a reduction of useful transmission bandwidth. The durationover which useful synchronization is maintained can be limited by thestability of the frequency references used in the transmitter andreceiver (e.g, provided by a local optical oscillator 302). Such aduration can be limited to a time scale of microseconds in the absenceof a high-stability frequency reference, after which timeresynchronization is generally performed. Integration of ultra-stablefrequency references such as derived from an integrated (e.g., local)optical clock in both the transmitter and receiver, can increaseduration of useful synchronization up to several seconds or more. In anexample, the local optical oscillator 302, such as shown in the examplesof FIGS. 5 through 6, can provide such an ultra-stable referencefrequency.

A frequency offset between the transmitted optical carrier and the localoptical oscillator 302 used at the receiver for coherent detection canbreak the orthogonality between subcarriers, such as resulting indetrimental inter-channel interference (ICI). To mitigate the potentialoffset, in one approach, lasers used in telecommunication can befrequency locked to an International Telecommunication Union (ITU)frequency standard with approximately 2.5-GHz accuracy. Thisconfiguration results in a frequency offset between the two opticalsources as large as 5 GHz. The down-converted baseband electronic signalcan be shifted by at least this frequency offset amount, necessitatinghigher bandwidth, and more expensive, electronics in the receiver thandictated by the bandwidth of the transmitted data alone. The presentinventors have recognized, among other things, that integrated opticalclocks in the transmitter and receiver locked to the same atomic ormolecular frequency reference can reduce such a frequency offset betweentransmitter and receiver from about GHz-range to about Hz-range,eliminating a need for unnecessary electronic bandwidth for basebandprocessing, and decreasing subcarrier separation.

Coherent Optical Detection

FIGS. 4A and 4B illustrate generally examples of coherent demodulatorsthat can include a heterodyne configuration in FIG. 4A and a homodyneconfiguration in FIG. 4B. Such coherent demodulators can be included asa portion of the communication systems shown in the examples of FIGS. 2through 3, such as included as a portion of the optical mixer 310 andcorresponding photo-detectors.

Digital information transfer generally involves efficient use of thefinite available bandwidth. For example, an optical fiber can supportdata transfer rates at or in excess of terabit per second (Tb/s) rates.The Shannon capacity generally places an upper limit on the number ofbits per symbol that can be successfully transmitted in the presence ofadditive white Gaussian noise (AWGN) for a given signal-to-noise ratioper symbol, and can be represented by ξ_(s),b _(max)=log₂(1+ξ_(s)).  EQN. 6

This limit, in turn, can govern the achievable spectral efficiency,which can be represented in bit/s/Hz. In an illustrative example, ifN_(ch) channels are available for data transmission, and the power ofthe transmitter (e.g., a transmitting laser) is divided equally amongthem, the total bit capacity of the system can be given by,

$\begin{matrix}{{b_{tot} = {N_{ch}{\log_{2}( {1 + \frac{\xi_{s}}{N_{ch}}} )}}},} & {{EQN}.\mspace{14mu} 7}\end{matrix}$which is a monotonically increasing function of N_(ch). Therefore, onetransmission strategy can be to distribute a given signal among allavailable channels, including all degrees of freedom (DOF) of thetransmitted optical field, such as including amplitude and phase. Datamodulation techniques utilizing optical phase (e.g., quadratureamplitude modulation (QAM)) can be referred to as coherent. In additionto increasing spectral efficiency, modulation of both the amplitude andphase, or equivalently the in-phase (I) and quadrature (Q) components ofa signal, can provide great flexibility in terms of available modulationformats.

Coherent modulation formats are generally received using coherentdetection of the transmitted signal for symbol recovery. In anillustrative example, down-conversion of a received optical passbandsignal to an electronic baseband signal can be performed such as throughmixing the optical passband signal (e.g., a modulated optical subcarriersignal that can be represented by E_(s,l)(t)), with a local oscillator(LO) such as using a square-law photodetector.

In an example, a receiver can be implemented in either a homodyne or aheterodyne configuration, or using one or more other configurations ortechniques.

In a homodyne configuration, the frequency of the LO is matched to thefrequency of the carrier so that the electronic output of thephotodetector is at baseband. In a heterodyne configuration, the LO andcarrier can differ in frequency, resulting in a down-converted signal atan intermediate frequency (IF) rather than at baseband. In such aheterodyne example, a subsequent down-conversion from IF to baseband canbe performed in the electrical domain such as using mixing with anelectronic LO.

In an illustrative example of a heterodyne receiver configuration, anoutput current of a balanced photodetector can be represented by,I _(het,l)(t)=R(|E ₁|² −|E ₂|²)  EQN. 8where R can represent the responsivity of each matched photodiode, andE_(i) can represent the electric field produced such as by mixingE_(s,l)(t) and E_(LO,l)(t) in the i^(th) photodiode. The values of E₁and E₂ can be related to the input signal and LO through the relations,

$\begin{matrix}{E_{1} = {\frac{1}{\sqrt{2}}\lbrack {{E_{s,l}(t)} + {j\;{E_{{LO},l}(t)}}} \rbrack}} & {{EQN}.\mspace{14mu} 9} \\{E_{2} = {\frac{1}{\sqrt{2}}\lbrack {{j\;{E_{s,l}(t)}} + {E_{{LO},l}(t)}} \rbrack}} & {{EQN}.\mspace{14mu} 10}\end{matrix}$such as where a 2×2 fiber coupler transfer function is assumed,

$\begin{matrix}{\frac{1}{\sqrt{2}}\begin{bmatrix}1 & j \\j & 1\end{bmatrix}} & {{EQN}.\mspace{14mu} 11}\end{matrix}$and the field of the local oscillator can be represented by,E _(LO,l)(t)=√{square root over (P _(LO,l))}e^(j[ω) ^(LO) ^(t+φ) ^(LO)^((t)])  EQN. 12where √{square root over (P_(LO,l))}, ω_(LO), and φ_(LO) canrespectively represent the average power, the angular frequency, and thephase of the LO. Insertion of the expressions for E₁, and E₂ intoI_(het,l)(t) yields,

$\begin{matrix}{{I_{{het},l}(t)} = {\frac{R}{2}{\begin{Bmatrix}{{\lbrack {{E_{s,l}(t)} + {j\;{E_{{LO},l}(t)}}} \rbrack\lbrack {{E_{s,l}^{*}(t)} - {j\;{E_{{LO},l}^{*}(t)}}} \rbrack} -} \\{\lbrack {{j\;{E_{s,l}(t)}} + {E_{{LO},l}(t)}} \rbrack\lbrack {{{- j}\;{E_{s,l}^{*}(t)}} + {E_{{LO},l}^{*}(t)}} \rbrack}\end{Bmatrix}.}}} & {{EQN}.\mspace{14mu} 13}\end{matrix}$which after algebraic simplification becomes,I _(het,l)(t)=2RIm{E _(s,l)(t)E* _(LO,l)(t)}+I _(sh,l)(t)  EQN. 14where I_(sh,l)(t) can represent the LO shot noise. The in-phase (I) andquadrature (Q) components of I_(het,l)(t) can be found explicitly, suchas by substitution of E_(s,l)(t) and E*_(LO,l)(t) into the aboveexpression, yieldingI _(het,l)(t)=2RIm{√{square root over (P _(r) P _(LO,l))}y _(Z,l)(t)e^(jφ(t)) +E _(sp,l)(t)E _(LO,l)(t)}+I _(sh,l)(t)where

$\begin{matrix}{{{y_{Z,l}(t)} = {\sum\limits_{k}^{\;}{\sum\limits_{m = 1}^{2}{x_{m,k}{c_{l\; m}( {t - {kT}_{s}} )}{\mathbb{e}}^{j\;{\phi{(t)}}}}}}},} & {{EQN}.\mspace{14mu} 16}\end{matrix}$andφ(t)=φ_(s)(t)−φ_(LO)(t).  EQN. 17

The real and imaginary parts of e^(jφ(t)) and y_(Z,l) can be representedexplicitly,e ^(jφ(t))=cos(φ)−j sin(φ)  EQN. 18y _(Z,l) =y _(I,l) +jy _(Q,l)  EQN. 19and the imaginary part of the resulting expression can be representedby,I _(het,l)(t)=2R√{square root over (P _(r) P _(LO,l))}[y _(I,l)sin(ω_(IF) t)+y _(Q,l) cos(ω_(IF) t)]+2R√{square root over (P_(LO,l))}{tilde over (E)} _(sp,l)(t)+I _(sh,l)(t)whereω_(IF)≡ω_(s)−ωLO  EQN. 21can represent the intermediate frequency and{tilde over (E)} _(sp,l)(t)≡Im{E _(sp,l)(t)e ^(j[ω) ^(LO) ^(t+φ) ^(LO)^((t)])}.  EQN. 22

In this heterodyne receiver configuration example, subsequent mixing atω_(IF) will separate the in-phase and quadrature component of thetransmitted signal. This information can be used to recover thetransmitted symbol.

In an example of a homodyne receiver configuration, the current at theoutputs of the balanced photodetectors can be represented by,I _(hom,I,l) =R√{square root over (P _(LO,l))}[√{square root over (P_(r))}y _(I,l)(t)+{tilde over (E)} _(sp,I,l)(t)]+I _(sh,I,l)(t),  EQN.23I _(hom,Q,l) =R√{square root over (P _(LO,l))}[√{square root over (P_(r))}y _(Q,l)(t)+{tilde over (E)} _(sp,Q,l)(t)]+I _(sh,Q,l)(t),  EQN.24where{tilde over (E)} _(sp,I,l)(t)=Re{E _(sp,l)(t)e ^(j[ω) ^(LO) ^(t+φ) ^(LO)^((t)])},  EQN. 25{tilde over (E)} _(sp,Q,l)(t)=Im{E _(sp,l)(t)e ^(j[ω) ^(LO) ^(t+φ) ^(LO)^((t)])},  EQN. 26which can be used in the receiver-side symbol decision process. In anexample, Bayesian estimation or one or more other estimation or decisiontechniques can be used for resolving symbol levels.

In an example, a transceiver can include both a coherent optical mixer,such as one or more of the heterodyne or homodyne configurationsdiscussed above, and an optical transmitter including an optical carriergenerator (e.g., an optical frequency comb generator). Individualoptical carriers from the optical frequency comb generator can becoupled to the coherent optical mixer such as to provide a synchronizedand stable local oscillator for downconversion.

Optical Discrete Fourier Transform

Data modulation can lead to spectral overlap between OFDM subcarriers(as shown in FIG. 1), such as precluding the use of optical narrowbandfiltering for individual subcarrier extraction, as is done in DWDM.Instead, a discrete Fourier transform (DFT) performed on the OFDMoptical information signal can be used to demultiplex an individualsubcarrier included in the OFDM optical information signal. In oneapproach, such a Fourier transform can be accomplished via processingthe OFDM signal electronically. Such an electronic bottleneckeffectively limits communication to Gigabit per second (Gb/s) datacommunication rates using generally-available electronic techniques.

The present inventors have recognized, among other things, that such anelectronic bottleneck can be circumvented such as performing DFT in theoptical domain (e.g., using an optical Fourier transform module 320 suchas shown in the example of FIG. 3), demultiplexing individualsubcarriers for subsequent parallel electronic processing at a lowersingle-carrier symbol rate. All-optical discrete Fourier transforms(ODFT), not to be confused with continuous Fourier transforms performedby lenses or free space propagation, can be performed with suitablesingle-mode star networks or using one or more other techniques orapparatus.

Optical discrete Fourier transforms operate continuously, as opposed tothose implemented electronically. Therefore, to obtain the spectralcomponents of a time series (e.g., the demultiplexed sub-carries of anOFDM signal), N time samples of the signal during a symbol period T canbe input in parallel into the optical DFT circuit. This can be achievedsuch as using optical serial-to-parallel conversion using amplitudesplitters, delay lines, and fast optical gating.

Local Optical Oscillator

FIG. 5 illustrates generally an example of at least a portion of anoptical oscillator that can be referenced to an atomic energy leveltransition or a molecular energy level transition and FIG. 6 illustratesgenerally an example of at least a portion of a frequency combgenerator, such as referenced to a signal provided by the apparatus ofthe example of FIG. 5. In an example, a local optical oscillator such asincluding the apparatus of FIGS. 5 and 6 can be used to provide areference frequency (e.g., 10 MHz, or one or more other frequencies)such as having a specified phase noise and stability.

One or more local optical oscillators can be integrated into a commonhardware platform with either (or both) an optical transmitter orreceiver, such as discussed in the examples above (e.g., one or moreportions of the examples of FIGS. 5 through 6 can be used as a firstlocal optical oscillator 102A as a portion of transmitter, a secondlocal optical oscillator 102B as a portion of a receiver, or in atransceiver as a local optical oscillator 302). Such local opticaloscillators can be configured to provide a reference frequency havingspecified phase noise, short-term, and long-term stability, such asreducing a frequency offset between a transmitter and a receiver, orreducing a need to correct for such offsets electronically. For example,one or more of an optical carrier generator 314 (e.g., an opticalfrequency comb generator), a complex modulator driver, an optical mixer310, an optical Fourier transform module 320, an electronic mixer, ananalog-to-digital converter, or one or more other portions of acommunication system can be referenced (e.g., “locked”) to a referencefrequency coupled electrically or optically from the local opticaloscillator.

In an example, the one or more local optical oscillators can provide thereference frequency using one or more of an atomic energy leveltransition or a molecular energy level transition. Such atomic ormolecular transitions can include using one or more species such ashydrogen, magnesium, ytterbium, mercury, silver, indium, aluminum,calcium, strontium, rubidium, iodine (I₂), acetylene (C₂H₂), carbondioxide (CO₂), or one or more other materials.

For example, I₂, C₂H₂, or CO₂ can be used in molecular form, atomicspecies such as Hg+, Yb+, Sr+, Ca+, In+, and Al+ can be used in an ionicform, and hydrogen, magnesium, ytterbium, mercury, calcium, rubidium orsilver can be used in a neutral atomic form. In an example, the one ormore local optical oscillators can be configured to generate a 10 MHzreference frequency signal output for respective portions of thecommunication system, or including one or more other referencefrequencies.

The frequency of transition of energy states of atomic or molecularmaterial in the local optical oscillator can be used for long termstabilization. For example, the following transition can be used,⁸⁷ Rb5S _(1/2)(F=2)→5D _(5/2)(F′=4)  EQN. 27

which can represent a two-photon transition frequency. An error signalsuitable for top-of-fringe locking can be derived from the transitionprofile. For example, such as shown in FIG. 5, a fiber coupledacousto-optic (AO) modulator can be coupled to an input of anErbium-doped Fiber Amplifier (EDFA), such as driven with a frequencymodulated (FM) signal from an FM source to impart a frequency dither ona spectroscopic probe beam. An output of EDFA can be provided to a fiberdoubler that can be coupled to a Rubidium cell (or a cell including oneor more other species from which an energy level transition will beprobed). In an illustrative example, such an AO modulator can include anAMF-55-1550-2FP+ such as available from Brimrose Corporation of America(Sparks, Md., USA).

In an illustrative example, a driving signal can be centered at about 55MHz, with modulation frequency and depth of 5 kHz and 150 kHz,respectively. A voltage output of Photo-multiplier Tube (PMT) can beused to detect a fluorescence at a first wavelength (e.g., about 420.3nm). Such a detected signal indicative of fluorescence can bedemodulated such as using a lock-in amplifier (e.g., an SR510® providedby Stanford Research Systems, Inc. of Sunnyvale, Calif., USA, or usingone or more other apparatus or techniques). The lock-in amplifier can bereferenced to a modulation frequency of AO modulator, such as to providean error signal used to lock the frequency of a fiber laser. Such atechnique can improve a long-term frequency stability of the localoptical oscillator.

In addition (or instead), the local optical oscillator can also bestabilized using a dimensionally-stable high finesse optical cavity tomeasure its frequency fluctuations which can then be removed via a highspeed feedback loop, such as shown in the lower portion of FIG. 5. Suchcavity-based stabilization can be used to provide short-term stability,complementing the atomic or molecular stabilization techniques discussedabove.

A laser's output can be routed to an AO modulator, such as driven by avoltage-controlled oscillator (VCO) and RF amplifier electronics, suchas to correct for frequency fluctuations measured by the ultra-lowexpansion (ULE) cavity. In an example, a portion of the AO modulatoroutput can be provided to a phase modulator (e.g., a fiber optic LithiumNiobate phase modulator), such as to place sidebands of the output oflaser at a desired offset (e.g., in an illustrative example, at about8.3 MHz). The phase-modulated optical energy can be coupled into a ULEglass Fabry Perot cavity of length about 7.75 centimeters (cm) andFinesse of about 150,000 (or more), although other types of cavitieswith different lengths and finesse values can be used. The transmissionthrough cavity can be monitored, such as using a low speed monitorphotodiode for diagnostic purposes. The reflected light from the ULEcavity can be coupled to high speed photodiode (e.g., in an illustrativeexample, a New Focus 1811® photodiode provided by Newport Corporation ofIrvine, Calif., USA), such as using a fiber optic circulator.

An error signal can be derived, such as by mixing an output of fiberoptic circulator with the drive signal of the phase modulator, such asto produce a dispersion shape signal. The dispersion shape signal can befed back to the laser, such as with a bandwidth up to or beyond about 10kHz, such as after filtering (e.g., using a low pass filter). In anexample, a portion of the error signal can be provided to the AOM driveelectronics such as for fast frequency correction with a bandwidth upinto the MHz range or beyond.

In the illustrative example of FIG. 6, an output of an optical frequencycomb (e.g., a fiber comb) can be used to provide a supercontinuum ofoptical energy peaks. In FIG. 6, connections that can be electrical areshown as broken lines, and connections that can be optical are shownwith solid lines. The output of the optical frequency comb can bestabilized, such as by locking to the optical output provided from theapparatus of FIG. 5 (“to Comb Laser”), such as to provide a highlystable, low jitter reference frequency for use in the communicationsystem discussed in the examples above. The fiber comb need not be theoptical frequency comb generator of the example of FIG. 3, but caninstead be a portion of the “precision optical oscillator” shown in FIG.3. The fiber comb transfers the stability of the stabilized opticaloscillator from the optical frequency regime into the RF frequencyregime.

Stabilization can be performed, for example, such as using a carrierenvelope offset (CEO) stabilization loop. The CEO can be used tostabilize the comb output relative to a specified frequency (e.g., zerofrequency). An octave spanning supercontinuum can be generated through anonlinear fiber. Such a supercontinuum has a characteristic that a beattone between a fundamental portion of the spectrum and the secondharmonic of the octave of the fundamental can represent an offset of thefrequency comb spectral components from “zero.” Such a tone output bythe frequency doubling interferometer can be mixed, such as using adigital phase detector, with an RF tone generated from disciplinedarbitrary waveform generator. A mixed signal output by the digital phasedetector can be fed into a controller, such as aProportional-Integral-Derivative (PID) controller. The PID controllercan adjust the pump power to the mode-locked laser of optical frequencycomb generator to stabilize the offset of the optical frequency combspectral components.

A second control loop can be used to stabilize the frequency combspacing of the optical frequency comb generator. Such stabilization canbe achieved, for example, by stabilizing the cavity length of amode-locked fiber laser of the optical frequency comb generator. Thefrequency comb can be coupled with a stabilized laser (e.g., the opticaloutput provided from the apparatus of FIG. 5 (“to Comb Laser”)), whichcan result in an RF beat tone signifying the frequency differencebetween the comb line and the stabilized laser. The RF beat tone can bedetected, such as using an InGaAs photoreceiver. This resulting RF beatcan be mixed against a second disciplined arbitrary waveform generatorin a digital phase detector. The output from the digital phase detectorcan be routed to a second controller, such as a PID controller, whichcan generate an error signal for adjusting the comb spacing. The errorsignal can be used, such as to control a piezo-actuated mirror inside amode-locked laser of the optical frequency comb generator. In anillustrative example, such a piezo-actuated mirror can provideapproximately 10 kHz of control bandwidth.

The present inventors have also recognized, among other things, that theuse of local oscillator stabilization techniques and apparatus above canimprove duration of useful synchronization between an opticaltransmitter and optical receiver. For example, such a synchronizedduration can be extended to durations as long as several seconds (orlonger), such as using a combination of short-term and long-termstabilization techniques. As a result, a better utilization oftransmission bandwidth over the optical channel can be possible becausedata communication can be interrupted less frequently by synchronizationsignals sent over the channel. It is believed that such stabilizationenables optical information transfer at rates approaching or exceedinghundreds of terabit per second (Tb/s) rates through a singlegenerally-available single mode optical fiber, such as when used toprovide one or more reference frequencies for a coherent optical OFDM(CO-OFDM) communication system.

In an example, synchronization information can be transferred betweentwo or more nodes in an optical communication system. For example, acommunication link such a free-space or fiber optic link can be used totransfer time or other synchronization information such as using pulsesor temporal markers according to a specified communication protocol. Inone approach, a pulse or marker arrival time can be measured orotherwise determined, and a temporal interval (e.g., a duration) betweenwhen the information (e.g., a marker or pulse) was transmitted and whenthe information was received. Applications for such synchronization caninclude government or civilian communications, such astelecommunications. Such synchronization can be accomplished eitherusing a free-space or fiber-optic link, or using one or more othercommunication media such as a wireless electromagnetic link. In anexample, one or more optical carriers or channels can be used for suchsynchronization, such as one or more carriers or channels in a CO-OFDMcommunication system.

VARIOUS NOTES & EXAMPLES

The above detailed description includes references to the accompanyingdrawings, which form a part of the detailed description. The drawingsshow, by way of illustration, specific embodiments in which theinvention can be practiced. These embodiments are also referred toherein as “examples.” Each of the non-limiting examples included hereincan stand on its own, or can be combined in any permutation orcombination with any one or more of the other examples. Such examplescan include elements in addition to those shown or described. However,the present inventors also contemplate examples in which only thoseelements shown or described are provided. Moreover, the presentinventors also contemplate examples using any combination or permutationof those elements shown or described (or one or more aspects thereof),either with respect to a particular example (or one or more aspectsthereof), or with respect to other examples (or one or more aspectsthereof) shown or described herein.

In the event of inconsistent usages between this document and anydocuments so incorporated by reference, the usage in this documentcontrols.

In this document, the terms “a” or “an” are used, as is common in patentdocuments, to include one or more than one, independent of any otherinstances or usages of “at least one” or “one or more.” In thisdocument, the term “or” is used to refer to a nonexclusive or, such that“A or B” includes “A but not B,” “B but not A,” and “A and B,” unlessotherwise indicated. In this document, the terms “including” and “inwhich” are used as the plain-English equivalents of the respective terms“comprising” and “wherein.” Also, in the following claims, the terms“including” and “comprising” are open-ended, that is, a system, device,article, composition, formulation, or process that includes elements inaddition to those listed after such a term in a claim are still deemedto fall within the scope of that claim. Moreover, in the followingclaims, the terms “first,” “second,” and “third,” etc. are used merelyas labels, and are not intended to impose numerical requirements ontheir objects.

Method examples described herein can be machine or computer-implementedat least in part. Some examples can include a computer-readable mediumor machine-readable medium encoded with instructions operable toconfigure an electronic device to perform methods as described in theabove examples. An implementation of such methods can include code, suchas microcode, assembly language code, a higher-level language code, orthe like. Such code can include computer readable instructions forperforming various methods. The code may form portions of computerprogram products. Further, in an example, the code can be tangiblystored on one or more volatile, non-transitory, or non-volatile tangiblecomputer-readable media, such as during execution or at other times.Examples of these tangible computer-readable media can include, but arenot limited to, hard disks, removable magnetic disks, removable opticaldisks (e.g., compact disks and digital video disks), magnetic cassettes,memory cards or sticks, random access memories (RAMs), read onlymemories (ROMs), and the like.

The above description is intended to be illustrative, and notrestrictive. For example, the above-described examples (or one or moreaspects thereof) may be used in combination with each other. Otherembodiments can be used, such as by one of ordinary skill in the artupon reviewing the above description. The Abstract is provided to complywith 37 C.F.R. §1.72(b), to allow the reader to quickly ascertain thenature of the technical disclosure. It is submitted with theunderstanding that it will not be used to interpret or limit the scopeor meaning of the claims. Also, in the above Detailed Description,various features may be grouped together to streamline the disclosure.This should not be interpreted as intending that an unclaimed disclosedfeature is essential to any claim. Rather, inventive subject matter maylie in less than all features of a particular disclosed embodiment.Thus, the following claims are hereby incorporated into the DetailedDescription as examples or embodiments, with each claim standing on itsown as a separate embodiment, and it is contemplated that suchembodiments can be combined with each other in various combinations orpermutations. The scope of the invention should be determined withreference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

The claimed invention is:
 1. A communication system, comprising: a localoptical oscillator configured to provide a reference frequency derivedfrom an atomic energy level transition or a molecular energy leveltransition; an optical carrier generator configured to generate at leasttwo specified optical carrier signals at least in part using thereference frequency provided by the local optical oscillator; a complexmodulator coupled to the optical carrier generator and configured tocoherently modulate the at least two specified optical carrier signalsusing respective baseband information signals to provide at least tworespective coherently-modulated optical subcarriers; a optical combinercoupled to the complex modulator and configured to combine the at leasttwo respective modulated optical subcarriers to provide a combinedoptical information signal.
 2. The communication system of claim 1,wherein the local optical oscillator is stabilized using a cavity modesupported by an optical cavity, and using one or more of an atomic or amolecular energy level transition.
 3. The communication system of claim1, wherein the respective baseband signals are specified to provideorthogonality between the respective coherently-modulated opticalsubcarriers.
 4. The communication system of claim 3, wherein respectivespectra of adjacent coherently-modulated optical subcarriers overlap. 5.The communication system of claim 1, wherein the optical carriergenerator comprises an optical frequency comb generator configured toprovide an optical frequency comb comprising at least two specifiedfrequency peaks, the at least two specified frequency peaks areseparated by a specified frequency offset.
 6. The communication systemof claim 1, comprising an electrical coupling between the local opticaloscillator and the optical carrier generator; and wherein the opticalcarrier generator is configured to receive the reference frequency viathe electrical coupling.
 7. The communication system of claim 1,comprising a complex modulator driver configured to provide therespective complex baseband information signals to the complexmodulator, including respective in-phase and quadrature signals; and anelectrical coupling between the local optical oscillator and the complexmodulator driver; and wherein the complex modulator driver is configuredto receive the reference frequency via the electrical coupling.
 8. Thecommunication system of claim 1, wherein the local optical oscillator isconfigured to provide a reference frequency derived from an atomicenergy level transition.
 9. The communication system of claim 1, whereinlocal optical oscillator is configured to provide a reference frequencyderived from a molecular energy level transition.
 10. A communicationsystem, comprising: a local optical oscillator configured to provide areference frequency derived from an atomic energy level transition or amolecular energy level transition; an optical Fourier transform moduleconfigured to receive an optical signal comprising at least twocoherently-modulated optical subcarriers and configured to provide atleast two coherent outputs respectively corresponding to the at leasttwo coherently-modulated optical subcarriers, at least in part using thereference frequency; and an optical mixer coupled to the optical Fouriertransform module and configured to coherently downconvert the at leasttwo coherent outputs corresponding to the at least twocoherently-modulated optical subcarriers to provide at least tworespective coherently-downconverted signals, at least in part using thereference frequency.
 11. The communication system of claim 10, whereinthe local optical oscillator is stabilized using a cavity mode supportedby an optical cavity, and using one or more of an atomic or a molecularenergy level transition.
 12. The communication system of claim 10,wherein respective spectra of adjacent coherently-modulated opticalsubcarriers overlap.
 13. The communication system of claim 10, whereinthe optical Fourier transform module comprises an optical discreteFourier transform module.
 14. The communication system of claim 10,comprising a balanced detector coupled to the optical mixer andconfigured to provide at least two respective electrical signalscorresponding to at least two coherently-downconverted signals.
 15. Thecommunication system of claim 14, wherein the optical mixer and thebalanced detector comprise a direct-conversion receiver configured toprovide a respective in-phase electrical signal and a respectivequadrature electrical signal for each of the at least two respectiveelectrical signals.
 16. The communication system of claim 14, comprisingan electrical mixer coupled to the balanced detector and configured toprovide a respective in-phase electrical signal and a respectivequadrature electrical signal for each of the at least two respectiveelectrical signals.
 17. The communication system of claim 14, comprisinga processor configured to receive information indicative of the at leasttwo respective electrical signals and configured to reconstructrespective baseband information signals; wherein the respective basebandsignals are specified to provide orthogonality between the respectivecoherently-modulated optical subcarriers comprising the optical signalreceived by the optical Fourier transform module.
 18. The communicationsystem of claim 10, wherein the local optical oscillator is configuredto provide a reference frequency derived from an atomic energy leveltransition.
 19. The communication system of claim 10, wherein localoptical oscillator is configured to provide a reference frequencyderived from a molecular energy level transition.
 20. A method,comprising: providing a reference frequency derived from an atomicenergy level transition or a molecular energy level transition;generating at least two specified optical carrier signals at least inpart using the reference frequency; coherently modulating the at leasttwo specified optical carrier signals using respective basebandinformation signals to provide at least two respectivecoherently-modulated optical subcarriers; and combining the at least tworespective modulated optical subcarriers to provide the combined opticalinformation signal.
 21. The method of claim 20, wherein providing thereference frequency includes stabilizing a local optical oscillatorusing a cavity mode supported by an optical cavity, and using one ormore of an atomic or molecular energy level transition.
 22. The methodof claim 20, wherein the respective baseband signals are specified toprovide orthogonality between the respective coherently-modulatedoptical subcarriers.
 23. The method of claim 22, wherein respectivespectra of adjacent coherently-modulated optical subcarriers overlap.24. The method of claim 20, wherein generating at least two specifiedoptical carrier signals includes generating an optical frequency combcomprising at least two specified frequency peaks, the at least twofrequency peaks separated by a specified frequency offset.
 25. A method,comprising: providing a reference frequency derived from an atomicenergy level transition or a molecular energy level transition;optically Fourier transforming a received optical signal comprising atleast two coherently-modulated optical subcarriers to provide at leasttwo coherent outputs respectively corresponding to the at least twocoherently-modulated optical subcarriers, at least in part using thereference frequency; and coherently downconverting the at least twocoherent outputs corresponding to the at least two coherently-modulatedoptical subcarriers to provide at least two respectivecoherently-downconverted signals, at least in part using the referencefrequency.
 26. The method of claim 25, wherein providing the referencefrequency includes stabilizing a local optical oscillator using a cavitymode supported by an optical cavity, and using one or more of an atomicor molecular energy level transition.
 27. The method of claim 25,wherein respective spectra of adjacent coherently-modulated opticalsubcarriers overlap.
 28. The method of claim 25, wherein the opticallyFourier transforming the received optical signal includes using adiscrete optical Fourier transform.
 29. The method of claim 25,comprising providing at least two respective electrical signalscorresponding to at least two coherently-downconverted signals using abalanced detector.
 30. The method of claim 25, comprising receivinginformation indicative of the at least two respective electricalsignals; and reconstructing respective baseband information signals;wherein the respective baseband signals are specified to provideorthogonality between the respective coherently-modulated opticalsubcarriers comprising the optically-Fourier-transformed optical signal.31. A system, comprising: a local optical oscillator configured toprovide a reference frequency derived from an atomic energy leveltransition or a molecular energy level transition; a carrier generatorconfigured to generate at least two specified carrier signals at leastin part using the reference frequency provided by the local opticaloscillator; a complex modulator coupled to the carrier generator andconfigured to coherently modulate the at least two specified carriersignals using respective baseband information signals to provide atleast two respective coherently-modulated subcarriers; wherein therespective baseband signals are specified to provide orthogonalitybetween the respective coherently-modulated subcarriers; and whereinrespective spectra of adjacent coherently-modulated subcarriers overlap.32. A method, comprising: providing a reference frequency derived froman atomic energy level transition or a molecular energy leveltransition; generating at least two specified carrier signals at leastin part using the reference frequency provided by the local opticaloscillator; coherently modulating the at least two specified carriersignals using respective baseband information signals to provide atleast two respective coherently-modulated subcarriers; wherein therespective baseband signals are specified to provide orthogonalitybetween the respective coherently-modulated subcarriers; and whereinrespective spectra of adjacent coherently-modulated subcarriers overlap.33. A system, comprising: a transmitter configured to generate amodulated information signal synchronized to a first reference frequencyprovided by a first local optical oscillator, the first local opticaloscillator configured to provide the first reference frequency at leastin part using an atomic energy level transition or a molecular energylevel transition; and a receiver configured to receive the modulatedinformation signal at least in part using a second reference frequencyprovided by a second local optical oscillator, the second local opticaloscillator configured to provide the second reference frequency at leastin part using an atomic energy level transition or a molecular energylevel transition; wherein a frequency offset between the first referencefrequency and the second reference frequency is specified to provide asymbol error rate of a received information signal below a specifiedthreshold for a duration of at least one second of information transferwithout requiring a resynchronization between the transmitter and thereceiver.
 34. The system of claim 33, wherein the transmitter is coupledto the receiver using at least one of a fiber optic link, a wirelesslink, or a conductive link.
 35. The system of claim 34, wherein thetransmitter is coupled to the receiver using a wireless link including afree-space optical link.
 36. A method, comprising: at a first location,generating a modulated information signal synchronized to a firstreference frequency provided by a first local optical oscillator, thefirst local optical oscillator configured to provide the first referencefrequency at least in part using an atomic energy level transition or amolecular energy level transition; at a second location, receiving themodulated information signal at least in part using a second referencefrequency provided by a second local optical oscillator, the secondlocal optical oscillator configured to provide the second referencefrequency at least in part using an atomic energy level transition or amolecular energy level transition; and wherein a frequency offsetbetween the first reference frequency and the second reference frequencyis specified to provide a symbol error rate of a received informationsignal below a specified threshold for a duration of at least one secondof information transfer without requiring a frequency offset estimationbetween the first reference frequency and the second referencefrequency.
 37. The method of claim 36, wherein the receiving theinformation signal includes receiving the information signal via atleast one of a fiber optic link, a wireless link, or a conductive link.38. The method of claim 37, wherein the receiving the information signalincludes receiving the information signal via a wireless link includinga free-space optical link.